The need exists for the fabrication of ceramic materials having improved electronic and magnetic properties which may be adjusted for a particular intended use. The present invention pertains to novel low electronic loss, voltage tunable ceramic materials exhibiting "adjustable" dielectric and magnetic properties. As used herein, the term "adjustable" means that a wide range of dielectric and magnetic properties can be attained by varying the amounts of the constituents of the composite. Hereafter, for the purpose of brevity, this type of material will be referred to as .mu./.di-elect cons. material, where .mu. refers to the magnetic property permeability, and .di-elect cons. refers to the electronic property permittivity.
The fabrication and use of BSTO and BSTO/MgO is known and has been practiced on previous occasions. BSTO has been known to be used for its high dielectric constant (ranging approximately from 200 to 6,000) in various antenna applications. This is set forth in the publication "Planar Microwave Electro-Optic Phase Shifters," Microwave Journal, Volume 35(6), (June 1992), by Richard W. Babbitt et al., which concluded that a need existed for materials having more desirable electronic properties.
To address this need, BSTO has been combined with additives such as MgO, thereby providing a ceramic ferroelectric composite material BSTO--MgO possessing superior electronic properties for use in antenna systems at both microwave and millimeter wave range frequencies. See, for example, U.S. Pat. No. 5,427,988, hereby incorporated by reference herein.
The dielectric properties of materials are evaluated through the measurement of the dielectric constant or relative permittivity (.di-elect cons..sub.r), tunability (T), and dissipation factor or loss tangent (tan .delta.). The magnetic properties of materials are measured at the same frequency range by measuring the relative permeability (.di-elect cons..sub.r).
Tunability may be defined as (dielectric constant with no applied voltage-dielectric constant with an applied voltage)/(dielectric constant with no applied voltage). For simplicity purposes, tunability can be represented as T EQU T=(X-Y)/X
wherein,
X=(dielectric constant with no applied voltage); and PA1 Y=(dielectric constant with an applied voltage).
The tunability of a material can range from 1-60% depending upon the composition of the materials employed.
Dielectric constant is related to the energy storage in the material; whereas, the loss tangent is related to the power dissipation in the same material. In general, the dielectric function is a complex quantity withs .di-elect cons.=.di-elect cons.'-i.di-elect cons."; and the loss tangent, tan .delta.=.di-elect cons."/.di-elect cons.'.
Permeability (.mu.) is a property of materials modifying the action of magnetic poles placed therein and modifying the magnetic induction resulting when the material is subjected to a magnetic field or magnetizing force. The permeability .mu. of a substance may be defined as the ratio of the magnetic induction in the substance to the magnetizing field to which it is subjected. The permeability of a vacuum is unity.
Although the fabrication of .mu./.di-elect cons. materials has been practiced in the past, the common application for such materials has been in absorbing (or reflecting) incident microwave energy in order to reduce electromagnetic interference. In such applications, a high dissipation factor is desired. For example, see both T. Yamamoto et al. in their publication, "Evaluation of ferroelectric/ferromagnetic composite by microcomposite designing," Ferroelectrics 95, 175 (1989); and J. V. Mantese et al. in their publication, "Application of effective medium theory to ferroelectric/ferrimagnetic composites with composition and frequency-dependent complex permittivities and permeabilities," J. Appl. Phys. 79 (3), (Feb. 1, 1996).
However, in applications where the microwave signal has to be launched into the active material through proper impedance matching with the incident medium, a high dissipation factor is not desirable. It is well known that the velocity of propagation of a microwave signal through a medium is dictated by the square root of the product of .mu. (permeability) and .di-elect cons. (permittivity). The greater the value of the product, the slower is the velocity of propagation of the wave through the medium. Moreover, the closer the ratio of .mu. to .di-elect cons. is to unity, the better the impedance matching of the circuit.
In current antenna designs, air is the dielectric medium and impedance matching is not easily achieved. However, by introducing the novel material of the present invention, comprising a ferrite/ferroelectric composite with matching permeability .mu. and permittivity .di-elect cons., one can design an antenna with not only a reduced path length, but where the incident energy can be introduced into the antenna with better impedance matching. Thus, this reduces the amount of energy lost or dissipated by reflection.
The present invention provides for improved materials having electronic and magnetic properties adjustable for use, for example, the invention is designed to provide a material suitable to create an impedance matching circuit in antenna applications. This material is particularly well suited to applications such as airborne antennas where impedance matching is required in order to miniaturize the apparatus. The present invention provides the advantage of a low dissipation factor (tan .delta.) in that the overall electronic loss of the composite is similar to the electronic loss of the ferroelectric constituent of the composite.